To put in my two cents I made this list, non-exhaustive, in no particular order:
- Traditional Books actually work. Many people will say that it is beneficial to use alternative methods and to try immersive tactics, etc, etc. Of course, supplementing academic-style language learning books is useful and something I would recommend, but supplementation is not replacement. It may be tempting to avoid the dryer, more potentially boring prospect of picking up a book and doing the exercises and listening to the conversations. But in the end, nothing can replace reading the grammar and slowly learning things in a pedagogical way.
- Consistency is key. I think that language learning is best done in shorter sessions, for example something like 45 minutes at a time might be optimum (for me). But on the other hand, to progress one has to have a lot of sustained contact and use with the language. For this reason, studying every day is recommended, in whatever fashion. And of course if every day isn't possible, then being as consistent as possible is a must.
- Don't postpone learning. I think many people put off learning today because they think they will have more time on the next day, or because they can somehow optimize the learning experience further (or buy another book, or download another podcast, etc, etc). Doing whatever you can today is always better then nothing, and the worst case is that you postpone learning so long that you have to re-learn something.
- You will make mistakes. Imagine anyone you know in your everyday life that is not a native speaker of your language. Store clerk, professor, doctor. Do they make mistakes? The answer is definitely yes. This should put it in perspective: everyone makes mistakes at every level of language learning, so the prospect of making mistakes should not scare you from speaking.
- It never ends. Related to the last point, the language learning journey never ends. Even if you are at the level where you need to be practically, you will still learn new words and new idioms that will push you even further. Combined with the fact that we all need refreshers to recall specific grammar points, we should be in the mindset that learning a foreign language (or more than one) is a continuous lifetime task (and that is a good thing, not bad!).